Surge arrestor for high voltage electric power applications

ABSTRACT

A surge arrestor for high voltage electric power applications comprises series-coupled uncontrolled discharge devices each of which is shunted through resistors by a string of avalanche diodes connected in series opposition, and a capacitor. Connected in series with each uncontrolled discharge device is a controlled discharge device with its igniter electrode coupled to the output of a control unit of the discharge device with the output of a current sensor connected to its input, the input of the current sensor being placed in series with one said string paralleled by the controlled discharge device. In electric apparatus rated for above 100 kV it is expedient to use a surge arrestor composed of a string of series-connected uncontrolled discharge devices and two controlled discharge devices.

United States Patent [191 Tolstov et al.

SURGE ARRESTOR FOR HIGH VOLTAGE ELECTRIC POWER APPLICATIONS Filed: July2, 1973 Appl. No.: 375,678

Related U.S. Application Data Continuation of Ser. No. 255,272, May 22,1972, abandoned.

U.S. Cl. 315/36, 315/190 1 Nov. 12, 1974 [51] Int. Cl. H02h 7/24 [58]Field of Search 315/190, 185, 188,36

[56] References Cited UNITED STATES PATENTS 5/1962 Dillon et al. 315/3612/1968 Cheerer 315/36 X Primary Examiner-Nathan Kaufman Attorney,Agent, or FirmHolman & Stern [57] ABSTRACT A surge arrestor for highvoltage electric power applications comprises series-coupleduncontrolled discharge devices each of which is shunted throughresistors by a string of avalanche diodes connected in seriesopposition, and a capacitor. Connected in series with each uncontrolleddischarge device is a controlled discharge device with its igniterelectrodecoupled to the output of a control unit of the discharge devicewith the output of a current sensor connected to its input, the input ofthe current sensor being placed in series with one said stringparalleled by the controlled discharge device. In electric apparatusrated for above 100 kV it is expedient to use a surge arrestor composedof a string of series-connected uncontrolled discharge devices and twocontrolled discharge devices.

5 Claims, 3 Drawing Figures PATENTEL HSV I 2 I974 SHEET 2 BF 3 PATENT E;rasv 1 2 I974 SHEET 3 OF 3 SURGE ARRESTOR FOR HIGH VOLTAGE ELECTRICPOWER APPLICATIONS This is a continuation of application I Ser. No.255,272, filed on May 22, 1972, which is now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to electric equipment and, more particularly, to surgearrestors for highvoltage electric power applications.

One of the key factors which ensure performance of electric apparatuswith an abundant margin of safety is stability of their operation in thepresence of voltage variations in A.C. or DC. supply mains.

An important factor, in particular, is the maximum voltage that can besafely tolerated by the insulation of apparatus, the magnitude of thisvoltage being determined either theoretically or experimentally. Fromthis point of view, the insulating strength of industrial electricequipment must be such as to be able to withstand maximum voltagesapplied thereto. In each concrete case, a selected value of insulatingstrength is governed by design and economic considerations and also bythe probability of maximum excess voltages.

2. Description of Prior Art Since in actual practice of operatingelectric installations maximum overvoltages are a rare occurrence, theelectric strength of their insulation is usually designed for voltagessomewhat lower than the maximum values, and protection from breakdownunder excess voltages which are liable to damage the insulation of aselected electric strength, is achieved by using various surge arrestorswhich are commonly arranged in parallel with the installation they areintended to protect.

The operating principle of known surge arrestors is that they areactuated as soon as overvoltages applied reach levels which may bedangerous for the insulation of the electric equipment (see, forexample, British Patent No. 795,307, class 39(l)d; US. Pat. Nos.2,611,108, cl. 315-36; 2,818,527, cl. 315-36; 2,492,850, cl. 315-181;2,659,839, cl. 315-150;

2,611,107, cl. 315-36; Foreign Pat. Nos. 754,646 and 1,056,716, cl. 21c,gr. 72; USSR lnventors Certificates Nos. 154,921, cl. 21c, gr. 72, and130,571, cl. 21d, gr. 2.

Surge arrestors manufactured in the Soviet Union and abroad'are put toperformance tests on AC. and DC. mains and also on devices forsuppressing switching transients, lightning arrestors, etc.

Analyzing the prior art practices in the USSR and other countries, itcan be inferred that surge arrestors have to meet clearly conflictingrequirements.

On the one hand, design and economic considerations dictate thatelectric equipment must have a minimum margin of insulating strengthdetermined by the upper limit at which the surge arrestor operates whenovervoltage arises; on the other, the lower limit of surge arrestoroperation must be able to ensure reliable performance of theinstallation without exceeding the permissible frequency of operation ofthe surge arrestor.

An additional complication is the fact that surge arrestors themselveswhich are generally a set of spark gaps also require the introduction ofcertain unpredictable corrections, since their switching voltages mayvary within a broad range depending on atmospheric conditions, designfeatures, etc.'

All the above considerations have made it imperative to appreciablyincrease the switching voltages and the margin of the insulatingstrength of electric apparatus.

invariably existing electric apparatus operate in con ditions when themargin of their insulating strength is used but very rarely, whichfactor raises to a considerable extent the operational costs of suchapparatus. Therefore, of major practical importance have becomesemiconductor diodes with an avalanche currentvoltagc characteristic atreverse voltages which characteristic permits dissipation of a certainportion of electric energy at these diodes. Studies of the applicabilityof a string of avalanche diodes connected in series opposition for surgesuppression have been conducted at the Krzhizhanovskiy Power Institute.

The operation of such a protective device is as follows. An externalvoltage applied to a string of avalanche diodes connected in seriesopposition is impressed mainly on the avalanche diodes in antiphase tothe voltage applied, because a voltage drop across the avalanche diodesconnected in phase with the voltage is in this case negligible. I

As soon as the voltage applied exceeds the sum of voltages determined bythe voltage-current characteristics at reverse voltages, current startsflowing in the circuit and its electric power is dissipated mainly atthe avalanche diodes on which reverse voltage is impressed.

The duration and magnitude of this surge current must not exceed thepermissible power value which is one of the basic parameters ofavalanche diodes of this type.

The effect of this protective arrangement lies in that during the entireovervoltage period, voltages of either polarity across a string ofavalanche diodes connected in series opposition are not above the sum ofvoltages at avalanche diodes coupled in antiphase to the voltageapplied. The level of voltages imposed on the protected electricinstallation is thus reduced.

, All the above is reported in the transactions of the KrzhizhanovskiyPower Institute for 1964-1966.

With a view to suppressing commutation transients in high-voltagethyristor converters, the companies AEG, Siemens, BBC (see B.B.M., 1969,56, No. 2 Thy ristorstromrichter fur kV Bruckengleichspannung") employeda surge arrestor composed of a string of avalanche diodes connected inseries opposition.

In such a series-opposition array of avalanche diodes howevercommutation overvoltages of both polarities imposed on thecontrolled-rectifier converter being protected are limited to the sum ofreverse voltages at series-connected avalanche diodes.

Besides, a surge protector may itself fail under some severeovervoltages imposed on the electric installation to be protected.

Theoretical and experimental investigations of electromagnetic processesin electric equipment in transient conditions and in the presence ofexcess voltages carried out by the applicants made feasible a newapproach to the development of surge arrestors to be described below. i

SUMMARY or THE INVENTION Switching transients in electric equipment canbe differentiated not only by the amplitude as was done in the priorart, but also from the energy point of view. Thus, for example,triggering of electric apparatus or commutation of valves inhigh-voltage converters may give rise to high-frequency (10 Hz)oscillations with the amplitudes reaching high levels even in thepresence of damping circuits, while their power is rather low. It hasbeen found possible to limit the amplitude and dissipate the power ofsuch excess voltages, for instance, by avalanche diodesv Such anapproach to the development of surge arrestors permits appreciablereduction in the margin of electric insulating strength of high-voltageelectric equipment because any transient voltage applied to thisequipment will stay below the voltage impressed upon the avalanchediodes.

However, apart from the above high-frequency oscillations, transientvoltages may contain also mediumfrequency (10 10 Hz) and low-frequencyl0 10 Hz) components the energy of which may exceed the powerdissipation rating of the voltage limiting device used.

An object of the present invention is to provide a device forsuppressing commutation and other overvoltages in high-voltage electricpower applications which use avalanche diodes in combination withspark-gap discharge devices.

A specific object of the invention is to provide a means for suppressionof surges in electric installations in the frequency range from 10 to 10Hz whereby overvoltages applied to the insulation of the electricequipment are reduced to a permissible level without damaging theprotective device itself.

In many cases differentiation of transient overvoltages with respect totheir energy enables one to substantially cut down the size and costs ofthe protected electric installation, the frequency of operation of a setof spark gaps remaining unchanged.

The concept presented abovehas been used for developing a surge arrestorfor high-voltage controlledrectifier sections of the KashiraMoscow D.C.transmission line, resulting in a 25 percent reduction in the voltageratings of the protected installations and consequent reduction of theirsize, weight and costs.

The surge arrestors designed and manufactured in accordance with theseprinciples have been put to a set of type and performance tests whichhave fully confirmed the validity of the selected approach. It should 7be pointed out that the use of such surge arrestors in the conditions ofthe mentioned transmission line would make it possible to bringsubstantial reduction in the voltage rating of each electric apparatusand consequently to cut down the weight and size of the equipment.

The invention is aimed at providing in a surge arrestor including atleast one controlled discharge device and at least one voltage limitingstring of at least two avalanche diodes connected in series-oppositionand provided for limiting voltage thereacross, at least one controlleddischarge device governed by a current sensor and connected in serieswith at least one uncontrolled discharge device, each of theuncontrolled and controlled discharge devices being shunted via adecoupler by said voltage-limiting string placed in parallel with acapacitor, while in series with at least one said string paralleled bysaid controlled discharge device is connected the input of a currentsensor with the output thereof being coupled through the control unitsto the igniter and working electrodes of said discharge device.

It is advantageous to make the said decoupler in the form of a linearresistor.

it is also advantageous that at least one discharge device beseries-connected with a surge current limiter.

lt is advantageous to make said surge-current limiter in the form ofanon-linear resistor.

It is also advantageous that at least one additional avalanche diode beconnected in phase with each avalanche diode of at least with one saidstring of diodes poled in series opposition.

The present invention will become more clear upon consideration of thefollowing description when taken in conjunction with the accompanyingdrawings in which concrete specialized terms are used. However. mostterms are used in a broader meaning, i.c. each particular term coversall equivalent elements operating in an identical manner and employedfor the same purposes as the present invention. Thus, a current sensormay be a current transformer or some other device.

BRIEF DESCRlPTlON OF THE DRAWlNG Additional objects and advantages ofthe invention may be better understood from the following descrip tionof a preferred embodiment thereof with reference to the accompanyingdrawings wherein:

FIG. 1 is a circuit diagram of a surge arrestor for high-voltageelectric power applications composed of uncontrolled discharge devicesin series with one controlled discharge device;

FIG. 2 is a circuit diagram of a surge arrcstor as in FIG. 1.,illustrating in detail units 2 and 8;

H6. 3 is a circuit diagram of a surge arrestor for high-voltage electricpower applications comprising uncontrolled discharge devices connectedin series relation with two controlled discharge devices, with twoadditional avalanche diodes being placed in series with each avalanchediode connected in series opposition.

DESCRIPTION OF THE PREFERRED lNVENTlVE EMBODIMENT The surge arrestorincludes series-connected uncontrolled discharge devices lalm (FIG. 1)each of which is paralleled via resistors 2a 2n by a string of avalanchediodes 3a 4a, 3b 4b 3n 4n connected in series opposition, and by arespective of capacitors 5a, 5b 5n. Connected in series relation withthe uncontrolled discharge devices is a controlled discharge device 6ashunting a string of avalanche diodes 3n 4n connected in seriesopposition, an igniter electrode 7 of the controlled discharge device 60being coupled to the output of a discharge device control unit 8 withits input connected to the output of a current sensor 9. while the inputof the current sensor is in series arrangement with the avalanche diodes3a 4a; 3b 4b 3n 4n.

The current sensor 9 has a secondary which is connected to the input ofcurrent integrator 10 of the unit 8 which controls the discharge device6a (FIG. 2). The surge arrestor is connected to an electric installationto be protected via terminals ll 11'. The current intogrator 10 is arectifier 12 built around diodes l3, l4, l5, l6.

The output of the rectifier 12 is connected across a storage capacitor17 through a resistor 18. The capacitor 17 is shunted by a resistor 19.

The output of the current integrator is coupled to the control electrodeof a thyristor via a decoupling diode 21, dynistor or switch-over diode22 and a limiting resistor 23, and to the cathode of the thyristor 20.

The cathode of the diode 21 and the anode of the dynistor 22 areconnected to the cathode of diode 24, while its anode connects acapacitor in shunt with a Zener diode 26, the anode of the diode 24 andthe free plate of the capacitor 25 being coupled with the cathode of thethyristor 20, and a DC. voltage source 27 is placed in parallel with theZener diode 26.

The thyristor 20 is connected into a circuit composed of a primary 28 ofa pulse transformer 29 and a capacitor with a DC. voltage source 32connected thereacross through a resistor 31.

A secondary 33 of the pulse transformer 29 is connected to the mainelectrode 34 and the igniter electrode 7 of the controlled dischargedevice 6a.

In normal operating conditions, the voltage applied to the installationdoes not exceed the total reverse voltage at the avalanche diodes of onepolarity: 3a 3n or 4a 4n (depending on the sign of voltage impressed onthe string of diodes), and, consequently, all the diodes of one polaritycannot be simultaneously driven into the avalanche portion of thecurrent-voltage characteristic. Current flowing through the primarywinding of the current transformer (current sensor 9) is the leakagecurrent of the avalanche diodes and is insufficient to produce a triggersignal for starting the discharge device 6a in the control unit 8.

In the presence of overvoltages, voltage across the terminals 11' -ll"rises up to the total reverse voltage at the diodes 3a 3n or 4a 4n,where upon all diodes of one polarity are driven into the avalancheportion of the current-voltage characteristic, and as a result, voltageat the terminals 11 11" ceases to grow. Surge current starts flowingthrough the string of avalanche diodes, the amplitude and duration ofwhich is determined by the electric characteristics of the voltageapplied and may be as high as tens and even hundreds of amperes.

The surge arrestor is capable of limiting voltage across the terminals 11' 1 1" in the presence of excess voltages until the energy held in theavalanche diodes 3a 3n or 4a 4n goes above the permissible value oruntil the current flowing through the avalanche diodes reaches thebreakdown level.

Then, the unit 8 delivers a signal to actuate the controlled dischargedevice 6a upon operation of which the uncontrolled discharge devices 1a1m operate and voltage from the terminals 11' 11" is removed.

Since after the avalanche diodes begin to operate in the avalancheportion of the current-voltage characteristic, voltage acrosstheterminals 11' 11" can be considered constant in the first approximation,the energy stored in the avalanche diodes will be proportional to theintegral of current flowing through these diodes.

The unit 8 performs periodic comparison of the energy stored in thestrings of avalanche diodes with the permissible value. In FIG. 2 themember responsible for such a comparison is the element 10 whichintegrates current passing via the strings of avalanche diodes by way ofthe storage capacitor 17 charged through the bridge rectifier 12employing diodes 13 16. The capacitor 17 is charged by current passingthrough the strings of avalanche diodes and the current from thesecondary winding of the current transformer (current sensor 9)delivered to the input of the integrator 10.

Voltage across the capacitor 17 is proportional to the energy in thestrings of avalanche diodes. The set-point voltageis the switchingvoltage of the dynistor 22. As soon as voltages across the capacitor 17and resistor 18 become equal to the switching voltage of the dynistor22, the dynistor starts to conduct and builds up a discharge path for anadditional capacitor 25 that has been charged from the source 27. Thecapacitors 17 and 25 have decoupling diodes 21 and 24 connectedtherebetween. Voltage across the capacitor 25 is regulated by means ofthe Zener diode 26 and its magnitude is substantially smaller than theswitching voltage of the dynistor 22. When discharging, the capacitor 25forms a pulse which goes via the decoupling diode 21 and the limitingresistor 23 to turn on the thyristor 20. Triggering of the thyristor 20to conduction causes discharge of the capacitor 30 that has been chargedfrom the source 32 through the resistor 31. The capacitor 30 dischargesthrough the primary winding 28 of the step-up pulse transformer 29 withitssecondary 33 connected to the igniter electrode 7 and the workingelectrode 34 of the discharge device 6a.

The control unit 8 is set so that the dynistor 22 switches over eitherin the case of maximum permissible power at the strings of avalanchediodes, or if current flowing through the avalanche diodes exceeds apredetermined level at least for some short time (current cut-off) whichalso prevents the breakdown of the avalanche diodes by overcurrents.

The resistor 18 of the integrator 10 serves for setting cut-off currentand the resistor 19 accomplishes periodic discharge of the capacitor 17.

If, under excessive voltage applied, the power dissipated in the stringsof avalanche diodes remains below the permissible level and the currentdoes not exceed the current cut-off value, the discharge device 6a isnot actuated. Then the surge arrestor affords its protective action onlyby limiting overvoltage across the terminals 11 11" of the protectedinstallation to the permissible value of the total reverse voltage atthe avalanche diodes. If, on the other hand, the overvoltageenergy islarger than the permissible power dissipated by the strings of avalanchediodes, the control unit 8 sends a trigger signal to the dischargedevice 6a. In this event the operation of the controlled dischargedevice is the proper action of the surge arrestor.

The surge arrestor described above (FIGS. 1 and 2) can be recommendedfor use in electric installations rated for about kV, irrespective ofthe polarity of peak voltages.

In order to provide overvoltage suppression in installations carryingvoltages above 100 kv, it is most advantageous to use a surge arrestorcomprising two serially connected uncontrolled discharge devices la andtwo controlled discharge devices 6a and 6b (FIG. 3).

Connected in series with each of the uncontrolled discharge devices 1a1m and controlled discharge devices 6a and 6b are surge current limitingelements 35a 35n (FIG. 3). Connection of the elements 350 35n in serieswith each discharge device 1a 1m and 6a 6b is chosen for design reasons.If necessary, all the elements 35a 3511 can be serially connected incommon to any of the discharge devices 1a 1m or 6a 6);.

Each of the avalanched diodes 3a 3n and 4a 4n coupled in seriesopposition is connected in series with two additional avalanche diodes3a 3"a 3n, 3"n and 4a, 4"a 4'n, 4"n.

Control of the controlled discharge devices 6a and 6b is accomplished byunits 8a and 8b identical to those described above.

As more severe overvoltages must be expected in electric installationsrated for above 100 kV it is advisable to increase the permissibledissipation power at the avalanche diodes of the surge suppressor whichis achieved by connecting additional avalanche diodes 3'a, 3"a 3'n, 3"nand 4'a, 4"a 4'n, 4"11.

Besides, the actuation of all the surge suppressors in such high-voltageinstallations causes the appearance of large short-circuit currentswhich would damage the equipment (e. g. surge suppressors, transformersand the like). The surge current limiters 35a 35:1 are included in thecircuitry with a view to reducing such short-circuit currents.

Finally, in order to increase the operation reliability of uncontrolleddischarge devices in the event of the terminals 11' 11" carryingvoltages above 100 kV, voltage across each of the discharge devices mustbe substantially raised. This is accomplished through using a largernumber of controlled discharge devices operating in the presence ofsevere overvoltages.

What is claimed is:

1. A surge arrestor for protecting apparatus in highvoltage electricpower applications, comprising: at least one string of at least twoavalanche diodes connected in series opposition, said at least onestring serving for limiting voltage across the electric apparatus beingprotected;

at least one uncontrolled discharge device;

at least one controlled discharge device having an ignitor and beinggoverned by a current sensor and connected in series configuration'withsaid at least one uncontrolled discharge device to form an addi tionalstring, said discharge devices protecting the strings of avalanchediodes connected in series opposition from excessive power dissipation;

a decoupler connected between said at least one string and each of saiddischarge devices so that said string is in shunt with said dischargedevices;

a capacitor connected in shunt with each discharge device of saidadditional string;

a current sensor having an output and connected in series with said atleast one string of avalanche diodes;

a control unit having an input and output and connected to said at leastone controlled discharge device with said input connected to the outputof said current sensor and said output coupled through a pulsetransformer to said ignitor, said controlled discharge device initiatingdischarge of all uncontrolled discharge devices connected in seriesconfiguration thereacross when said control unit sends a trigger signalto said controlled discharge device,

2. A surge arrestor as of claim 1, wherein said decoupler comprises aresistor.

3. A surge arrestor as of claim 2, further comprising a surge currentlimiter connected in series with at least one said discharge device.

4. A surge arrestor as of claim 3, wherein said surge current limitercomprises a nonlinear resistor.

5. A surge arrestor for high-voltage electric power applications as ofclaim 4, further comprising at least one additional avalanche diodeconnected in series aiding with each of the avalanche diodes connectedin se ries opposition, said additional avalanche diodes serving toincrease permissible power dissipation in a string of avalanche diodesconnected in series opposition.

1. A surge arrestor for protecting apparatus in high-voltage electricpower applications, comprising: at least one string of at least twoavalanche diodes connected in series opposition, said at least onestring serving for limiting voltage across the electric apparatus beingprotected; at least one uncontrolled discharge device; at least onecontrolled discharge device having an ignitor and being governed by acurrent sensor and connected in series configuration with said at leastone uncontrolled discharge device to form an additional string, saiddischarge devices protecting the strings of avalanche diodes connectedin series opposition from excessive power dissipation; a decouplerconnected between said at least one string and each of said dischargedevices so that said string is in shunt with said discharge devices; acapacitor connected in shunt with each discharge device of saidadditional string; a current sensor having an output and connected inseries with said at least one string of avalanche diodes; a control unithaving an input and output and connected to said at least one controlleddischarge device with said input connected to the output of said currentsensor and said output coupled through a pulse transformer to saidignitor, said controlled discharge device initiating discharge of alluncontrolled discharge devices connected in series configurationthereacross when said control unit sends a trigger signal to saidcontrolled discharge device.
 2. A surge arrestor as of claim 1, whereinsaid decoupler comprises a resistor.
 3. A surge arrestor as of claim 2,further comprising a surge current limiter connected in series with atleast one said discharge device.
 4. A surge arrestor as of claim 3,wherein said surge current limiter comprises a nonlinear resistor.
 5. Asurge arrestor for high-voltage electric power applications as of claim4, further comprising at least one additional avalanche diode connectedin series aiding with each of the avalanche diodes connected in seriesopposition, said additional avalanche diodes serving to increasepermissible power dissipation in a string of avalanche diodes connectedin series opposition.